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Cellular softening mediates leukocyte demargination and trafficking, thereby increasing clinical blood counts

  1. Wilbur A. Lama,b,c,d,e,2
  1. aThe Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology & Emory University, Atlanta, GA 30332;
  2. bDepartment of Pediatrics, Division of Pediatric Hematology/Oncology, Aflac Cancer Center and Blood Disorders Service of Children’s Healthcare of Atlanta, Emory University School of Medicine, Atlanta, GA 30322;
  3. cWinship Cancer Institute, Emory University, Atlanta, GA 30322;
  4. dParker H. Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, GA 30332;
  5. eInstitute for Electronics and Nanotechnology, Georgia Institute of Technology, Atlanta, GA 30332;
  6. fDepartment of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, WI 53706;
  7. gGeorge W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332;
  8. hPhysics Faculty, The Evergreen State College, Olympia, WA 98505

  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved January 12, 2016 (received for review May 6, 2015)

Significance

Clinical hematologists have long known that antiinflammatory glucocorticoids such as dexamethasone and blood pressure-supporting catecholamines such as epinephrine cause leukocytes to demarginate from the vascular wall and microvasculature into the main circulation, significantly elevating the effective white blood cell count. Canonically, this has been attributed to down-regulation of adhesion molecules such as selectins, but we show that a purely mechanical phenomenon caused by leukocyte softening plays a major role as well. Our work provides an answer to an old hematological problem and reveals a mechanism in which the immune system simply alters leukocyte stiffness to regulate leukocyte trafficking. This has clinically relevant implications for the inflammatory process overall as well as for hematopoietic stem cell mobilization and homing.

Abstract

Leukocytes normally marginate toward the vascular wall in large vessels and within the microvasculature. Reversal of this process, leukocyte demargination, leads to substantial increases in the clinical white blood cell and granulocyte count and is a well-documented effect of glucocorticoid and catecholamine hormones, although the underlying mechanisms remain unclear. Here we show that alterations in granulocyte mechanical properties are the driving force behind glucocorticoid- and catecholamine-induced demargination. First, we found that the proportions of granulocytes from healthy human subjects that traversed and demarginated from microfluidic models of capillary beds and veins, respectively, increased after the subjects ingested glucocorticoids. Also, we show that glucocorticoid and catecholamine exposure reorganizes cellular cortical actin, significantly reducing granulocyte stiffness, as measured with atomic force microscopy. Furthermore, using simple kinetic theory computational modeling, we found that this reduction in stiffness alone is sufficient to cause granulocyte demargination. Taken together, our findings reveal a biomechanical answer to an old hematologic question regarding how glucocorticoids and catecholamines cause leukocyte demargination. In addition, in a broader sense, we have discovered a temporally and energetically efficient mechanism in which the innate immune system can simply alter leukocyte stiffness to fine tune margination/demargination and therefore leukocyte trafficking in general. These observations have broad clinically relevant implications for the inflammatory process overall as well as hematopoietic stem cell mobilization and homing.

Footnotes

  • ↵1M.E.F. and D.R.M. contributed equally to this work.

  • ↵2To whom correspondence should be addressed. Email: wilbur.lam{at}emory.edu.

  • Author contributions: M.E.F., D.R.M., A.K., C.T.T., R.B., Y.S., M.J.R., N.A.S., T.A.S., M.D.G., and W.A.L. designed research; M.E.F., D.R.M., A.K., C.T.T., R.B., K.C., R.G.M., A.L., E.A.T., and T.A.S. performed research; M.E.F., D.R.M., C.T.T., R.B., K.C., A.L., T.A.S., M.D.G., and W.A.L. analyzed data; and M.E.F., D.R.M., A.K., C.T.T., R.B., R.G.M., E.A.T., T.A.S., M.D.G., and W.A.L. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1508920113/-/DCSupplemental.

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